U.S. patent application number 14/016406 was filed with the patent office on 2014-06-19 for methods for producing textured electrode based energy storage device.
This patent application is currently assigned to NANOSCALE COMPONENTS, INC.. The applicant listed for this patent is NANOSCALE COMPONENTS, INC.. Invention is credited to Robert W. Grant, Matthew Sweetland.
Application Number | 20140166491 14/016406 |
Document ID | / |
Family ID | 44066963 |
Filed Date | 2014-06-19 |
United States Patent
Application |
20140166491 |
Kind Code |
A1 |
Grant; Robert W. ; et
al. |
June 19, 2014 |
METHODS FOR PRODUCING TEXTURED ELECTRODE BASED ENERGY STORAGE
DEVICE
Abstract
This method enables the use of nanowire or nano-textured forms
of Polyaniline and other conductive polymers in energy storage
components. The delicate nature of these very high surface area
materials are preserved during the continuous electrochemical
synthesis, drying, solvent application and physical assembly. The
invention also relates to a negative electrode that is comprised of
etched, lithiated aluminum that is safer and lighter weight than
conventional carbon based lithium-ion negative electrodes. The
invention provides for improved methods for making negative and
positive electrodes and for energy storage devices containing them.
The invention provides sufficient stability in organic solvent and
electrolyte solutions, where the prior art processes commonly fail.
The invention further provides stability during repetitive charge
and discharge. The invention also provides for novel microstructure
protecting support membranes to be used in an energy storage
device.
Inventors: |
Grant; Robert W.; (Camden,
ME) ; Sweetland; Matthew; (Bedford, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NANOSCALE COMPONENTS, INC. |
Hudson |
NH |
US |
|
|
Assignee: |
NANOSCALE COMPONENTS, INC.
Hudson
NH
|
Family ID: |
44066963 |
Appl. No.: |
14/016406 |
Filed: |
September 3, 2013 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13018586 |
Feb 1, 2011 |
8529746 |
|
|
14016406 |
|
|
|
|
PCT/US10/58418 |
Nov 30, 2010 |
|
|
|
13018586 |
|
|
|
|
61265167 |
Nov 30, 2009 |
|
|
|
61353500 |
Jun 10, 2010 |
|
|
|
Current U.S.
Class: |
205/59 ;
205/234 |
Current CPC
Class: |
H01M 4/1395 20130101;
C25F 3/04 20130101; H01M 4/0461 20130101; C25D 13/12 20130101; H01G
11/86 20130101; H01M 4/0452 20130101; H01G 9/0029 20130101; H01G
11/24 20130101; H01M 2/1653 20130101; Y02E 60/10 20130101; C25D
5/44 20130101; C25D 3/42 20130101; C25D 9/02 20130101; C25D 11/00
20130101; H01G 11/52 20130101; H01G 9/042 20130101; H01M 4/137
20130101; H01M 4/366 20130101; H01M 4/0466 20130101; H01M 10/052
20130101; C09D 5/24 20130101; Y02E 60/13 20130101; C09D 179/02
20130101; H01G 11/48 20130101; H01M 10/0525 20130101; C25D 5/40
20130101; H01M 4/134 20130101; H01M 4/1399 20130101 |
Class at
Publication: |
205/59 ;
205/234 |
International
Class: |
H01G 9/00 20060101
H01G009/00; H01M 4/04 20060101 H01M004/04 |
Claims
1. A method for making a lithiated electrode comprising lithiating
an aluminum electrode wherein a reducing current is applied in a
moisture free organic electrolyte solution comprising a lithium
salt.
2. The method of claim 1, wherein the aluminum electrode comprises
a magnesium containing aluminum alloy.
3. The method of claim 1, wherein the aluminum alloy comprises
between 1 and 20 atom percent magnesium.
4. The method of claim 1, wherein the aluminum alloy comprises at
least 3 atom percent magnesium.
5. The method of claim 1, wherein the reducing current is about 5
ma/cm.sup.2.
6. The method of claim 1, wherein the lithium salt is selected from
the group consisting of LiSO.sub.4, LiBF.sub.4 and LiClO.sub.4.
7. The method of claim 1, wherein the lithium salt is maintained in
a concentration of at least about 0.5M.
8. The method of claim 1, wherein the lithium salt is maintained in
a concentration of at least about 1.0M.
9. The method of claim 1, wherein the organic electrolyte solution
comprises propylene carbonate and dimethoxyethane (PC/DME).
10. The method of claim 1, wherein lithium is added to the aluminum
electrode in an amount of about 5 and 30 atom percent.
11. The method of claim 1, wherein the moisture conditions are less
than -40 degrees dew point air.
12. The method of claim 1, wherein the aluminum electrode is
electrochemically etched prior to lithiation.
13. The method of claim 1, wherein the electrochemical etching is
achieved by adding a 1 M HCl, 5% ethylene glycol solution at room
temperature with an oxidizing current.
14. The method of claim 1, wherein the oxidizing current is at
least 0.02 A/cm3.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 13/018,586, filed Feb. 1, 2011, which is a continuation of
International Application No. PCT/US10/58418, which designated the
United States and was filed on Nov. 30, 2010, published in English,
which claims the benefit of U.S. Provisional Application No.
61/265,167, filed Nov. 30, 2009 and U.S. Provisional Application
No. 61/353,500, filed Jun. 10, 2010. The entire teachings of the
above applications are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] In 1986, Alan MacDiamid won a Nobel Prize for his work in
conductive polymers. In his U.S. Pat. No. 4,940,640, MacDiamid
disclosed the chemical nature of polyaniline and the five possible
states of oxidation. At that time, polyaniline was synthesized by
chemical routes, and later stimulated into various electrical
states by electrochemistry. In the early 1990's, researchers such
as V. Gupta and M. Pasquali, published the construction of
polyaniline and polypyrrole conductive nanowires by electrochemical
synthesis in acids such as Hydrochloric, Sulphuric and Perchloric
acids. These films had very high electrolytic performance, but were
fairly unstable physically and electrically, decomposing in use.
Over the years since then, gradual increases in chemical stability
have been achieved by including dopant donors such as sulphonic
acid. In this case, the RSO.sub.3.sup.- anion is synthesized into
the polyaniline, creating a more stable material, albeit at the
expense of slower growth or poorer adhesion at the electrode. Since
the initial work, many papers have been published regarding
electrochemical and chemical synthesis; however, there has been a
distinct lack of implementation of the most useful forms of
conductive polymers due to the delicate nature of "brush" like
features. The loose fibers or nano-texture can mat down during
assembly to the opposing electrode, reducing most of the useful
surface area. The current collectors for the studies have been
primarily noble metals, which are costly and therefore, limiting to
commercial applications. Lack of adhesion to other less noble
metals primarily due to oxidation has also limited their use.
Commercial applications of conductive polymers have therefore been
via the bulk chemical (granular) synthesis route, where the
available specific surface areas are as much as ten times less as
compared to the more delicate electrochemically grown
nano-texture.
[0003] Therefore, it is an object of the present invention to
provide a method for producing nano-textured conductive polymers on
non-noble metal electrodes wherein the delicate nature of these
very high surface area materials is preserved during the continuous
electrochemical synthesis, drying, solvent application and physical
assembly and repetitive charge and discharge.
SUMMARY OF THE INVENTION
[0004] The invention relates to the synthesis of a conductive
polymer onto a non-noble metal electrode. High power and high
energy organic nanowire ultracapacitors (supercapacitors) or
batteries are made from low-cost materials, and produced by an
automated continuous sheet process. The continuous method of
manufacture includes electrochemical techniques: One technique is
especially adapted to initiate a favorable seed layer or template
for growth of the conductive polymer that has good adhesion and
electrical properties; another technique provides rapid growth upon
the seed layer or template while propagating favorable morphology
and electrical properties. The invention also relates to a negative
electrode that is comprised of etched, lithiated aluminum that is
safer and lighter weight than conventional carbon based lithium-ion
negative electrodes. The invention provides for improved methods
for making negative and positive electrodes and for energy storage
devices containing them. The invention provides sufficient
stability in organic solvent and electrolyte solutions, where the
prior art processes commonly fail. The invention further provides
stability during repetitive charge and discharge. The invention
also provides for novel microstructure protecting support membranes
to be used in an energy storage device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] The foregoing and other objects, features and advantages of
the invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
[0006] FIG. 1: Metal ribbon.
[0007] FIG. 2: Electrode layered structure.
[0008] FIG. 3: Electrochemical synthesis bath.
[0009] FIG. 4: Electron micrograph image of nanowire product.
[0010] FIG. 5: CV cycle testing of nanowire electrode.
[0011] FIG. 6: Manufacturing scheme.
[0012] FIGS. 7A-7C: Spacer/layer positive and negative
electrodes.
[0013] FIGS. 8A-8C: Stack of alternating electrode and spacer
layers.
[0014] FIG. 9: Ultracapacitor performance as a function of pH and
dopant level.
[0015] FIGS. 10A and 10B: Discharge curves.
[0016] FIG. 11: Discharge curve at constant current of 2.5
mA/cm.sup.2.
[0017] FIG. 12: Thermal model, showing adequacy of 4 micron
substrate thickness.
[0018] FIG. 13: Cycle testing of polyaniline on a base metal
electrode.
[0019] FIG. 14: Illustrates performance of a half cell arrangement
of polyaniline electrochemically synthesized onto graphite scrubbed
Aluminum 1145.
[0020] FIG. 15: Illustration of cyclic voltammetry of a
Ni/4-APA/PANi electrode, as made in Example 1, response in aqueous
low pH solutions.
[0021] FIGS. 16 and 17: Illustration of cyclic voltammetry of a
Ni/C/PANi electrode, as made in Example 2, response of a 20 C film
in aqueous low pH solutions and nonaqueous solutions.
[0022] FIG. 18: Illustration of nonaqueous discharge capacity (mAh)
for a Ni/C/PANi electrode as a function of total Coulombs of
aqueous growth.
[0023] FIG. 19: Illustration of a guide roller of the invention
with a film pulled over it. The line of tension is illustrated
together with the preferred fluid gap.
[0024] FIG. 20: Illustration of a perforated roller that can be
used to minimize shear forces during processing.
[0025] FIG. 21: Illustrates an assembly wherein 5 layers of
electrode plates (3 per layer) are assembled with guide rollers for
directing a film to each layer.
[0026] FIG. 22: Illustrates a preferred drying tunnel of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0027] The invention relates to improved electrodes comprising a
support, linker layer, and conductive polymers on one, two or more
sides of the support as shown in FIG. 2. The electrode can
preferably be in the shape of a conductive ribbon or similar sheet
having conductive polymer on both sides. The methods of the
invention can be used to make the improved electrodes continuously
or semi-continuously, as desired, and allow for electrode creation
via chemical and electrochemical process steps which can be applied
simultaneously to both sides of the substrate.
[0028] The invention also relates to an innovation in chemistry,
whereby the incorporation of high energy density (high surface
morphology), successful synthesis on commodity metals, and
successful transfer to organic electrolytes allows for low cost,
high density energy storage.
[0029] The invention further relates to improved energy storage
manufacturing. The fundamental innovation in energy storage
manufacturing is the ability to synthesize nano-textured material
in a continuous, non-contact process. The non-contact
electrochemical synthesis allows the use of exceptionally thin
substrates due to the low forces and tensions required by the
non-contact process. The use of such thin substrates in turn lends
itself to optimization through stacked design as a means of
minimizing electrical path length and corresponding resistance. Low
cost and high specific energy are achieved by using very small
amounts of lightweight, commodity-priced metals.
[0030] The electrode comprises a support, a linker or bifunctional
agent and a conductive polymer having a very high surface area. The
electrode can be any shape or size. However, the electrode is
preferably a thin sheet or plate. Ribbons of thin electrode, where
the length far exceeds the width, are particularly preferred as
they facilitate an efficient continuous processing. In this
embodiment, the width is selected to correlate to the width of the
finished product. The ribbon can then be easily cut to the length
of the finished product for final assembly. FIG. 1 illustrates such
a ribbon. The embodiment in FIG. 1 shows optional sprocket holes
along the length of the ribbon.
[0031] The support should be sufficiently thick as to provide
meaningful support and durability to the device, thereby protecting
the polymer structure from physical stresses which can be caused by
bending and flexing. However, the support should not be so thick as
to be unjustly heavy for the contemplated use. In many
applications, the thickness of the support can be less than about
100 microns, preferably less than about 50 microns and more
preferably less about 10 microns. Further, the support can
generally be greater than about 4 microns.
[0032] The support can be made of a large variety of materials. It
is understood that the support comprises a conductive material or
metal. In an alternative embodiment, the conductive material can
coat a substrate (which need not be conductive), such as polymers,
plastics, cellulosic materials, resins, glass, ceramics, metals,
graphite and the like. A polymeric support can be a polyester, such
as polyethylene terephthalate, PET.
[0033] The electrode can be made of, contain or be coated with a
conductive metal, e.g. Titanium, Aluminum, Nickel, Stainless Steel,
Tin, Gold, and Platinum. Non-noble metals are preferred due to
their lower cost. Aluminum is preferred because of its low cost and
availability. Nickel is preferred due to its low cost and relative
chemical stability of its surface.
[0034] In this example, the thickness of the metal is about 2000
Angstroms thick each side on a 4 micron or greater thick polymer
sheet such as polyester. Other polymers such as treated BOPP
(biaxially oriented polypropylene) can be used, but polyesters have
good to excellent natural adhesion properties with regard to
evaporated metals. Sprocket holes can be incorporated along one or
both lengths of the ribbon to facilitate transport through the
process.
[0035] In one embodiment of the invention, the process includes
deposition of an evaporated conductive metal on a substrate or an
electroplating step to produce the support. Such processes are well
known in the art. Preferably, thin films or foils of conductive
materials are preferred.
[0036] In a preferred embodiment of the invention, a conductive and
exfoliable (sheets can be rubbed off by friction) form of carbon
such as graphite or graphene (cgg) is scrubbed, or buffed, onto the
surface of the electrode substrate or base metal electrode base.
The exfoliable carbon becomes imbedded in the electrode substrate
and provides a conductive and chemically stable support (for
linking between the metal and polyaniline or PANi) for the
electrochemically synthesized polymer. Although conductive polymers
have been synthesized over hard, polished carbon substrates, it is
here discovered that conductive polymers can be electrochemically
synthesized onto graphite micro-particles on base metals while
maintaining the electrical performance of a film grown onto noble
metal substrates. The graphite micro-particles create a link
between the base metal and the PANi film to afford a method for
attaching PANi to surfaces that have up to this date proven
extremely difficult to achieve in non aqueous environments.
[0037] Graphite or graphene can be applied to the thin metal
electrode by a buffer pad at high speed, either in a random,
circular or unidirectional pattern. For example, a moving belt or
orbital buffing device (e.g. 4000-11000 rpm at 1-20 psi) can be
used. The thickness of the carbon layer so produced in 1 to 10
seconds of application can be uniform, continuous and 10 to 100
nanometers in thickness.
[0038] An example of a suitable buffering process can be adapted
from U.S. Pat. No. 6,511,701, which is incorporated herein by
reference. In the process, dry carbon particles can be applied to a
metal, preferably aluminum, surface uniformly. In the present
invention a buffing pad can be moved in the plane of the substrate
parallel to the substrate surface, or surfaces. The random orbital
motion of the pad in the present invention is carried out with its
rotational axis perpendicular to the substrate or web. Since the
electrode is moving in the longitudinal direction, a simple linear
or back and forth motion in the latitudinal direction will also
suffice. The coating does not require the heating step described
therein. Further, the units can be modular such that by stacking
multiple units on top of each other, both sides of the substrate
can be coated such that a first side is coated in a first unit and
the film is then directed (up or down) to a second unit where the
opposing side is coated. A hardened stainless steel pressure plate
is used to apply a uniform and controllable pressure to the
film/vibrator pad interface. The modular units are sealed and kept
under a negative pressure to keep the graphite powder
contained.
[0039] The electrochemically grown polymerization begins
immediately on this surface, produced over metals, such as
aluminum. The adhesion of the synthesized polymer on this carbon
imbedded surface on aluminum, as measured by a cellophane tape
pull-off test, is good, and the electrical stability of the film is
good through 6500 cycles in non-aqueous electrolyte. It is
preferred that the carbon micro particulate (less than a continuous
film) be formed on as received oil-free aluminum. Cleaning the
aluminum with e.g. phosphoric acid or sodium hydroxide will alter
the native oxide and alter the performance of the film during
synthesis initiation due to the heavy aluminum oxidation current
produced. If the aluminum is fresh (without any oxide), then a
controlled partial oxidation can be achieved with steam, hot air,
etc., treatments known in the art. Preferably the native oxide is
established prior to carbon imbedding, but it is possible to
oxidize the aluminum in a second step following carbon imbedding.
Preferably the aluminum may be ensured to be oil-free via exposure
to corona discharge.
[0040] In case of the use of a chemical linker, the growth of the
conductive polymer on non-noble metals can be described as five
steps: 1) The metal surface of the support is stripped bare to (or
provided as) a non-oxidized metal; 2) Hydroxylation is achieved in
order to facilitate a chemical reaction; 3) A bifunctional agent is
linked onto the surface (A chemical having or involving two
functional groups or binding sites); 4) Polymer growth is initiated
onto the linker or bifunctional layer; and 5) Polymer growth is
proliferated. A lesser set of steps may be desired for noble metal
applications. A more detailed explanation follows:
[0041] 1) In order to clean the substrate down to bare metal,
either mechanical means (e.g. brushing or polishing) or chemical
means such as 3:1 room temperature HCl:HNO.sub.3 in water solution
for a number of seconds (e.g. 15 seconds) is performed. Other
immersion times such as five seconds or 200 seconds are also
possible, but too little time does not clear the oxide and too much
time may remove too much metal. A corona discharge may also be
applied in order to remove hydrocarbon material.
[0042] 2) In order to Hydroxylate Nickel, for example, the metal
surface is immersed in 1 molar KOH with several (e.g. 2 or more,
such as between 3 and 10) cyclic voltammetry cycles between 0.2 and
0.5 volts re. Ag/AgCl. The rate can be conveniently applied at 20
mV/sec in a basic solution, such as a 2M aqueous NaOH solution.
There are other ways to Hydroxylate Nickel, for example with the
use of bases at elevated temperature. The CV method is preferred
since it avoids conditions where excessive oxidation takes place.
Aluminum, Titanium and other metals can also be hydroxylated by
treatments with various bases such as Sodium Carbonate, metal
Hydroxides, etc. The hydroxylated product can be rinsed, if
desired.
[0043] 3) In order to prepare a stable and favorable surface to
polymer growth, a chelating or bifunctional agent-containing
solution, such as Phthalic or Phosphonic acid can be used.
Bifunctional agents include compounds which will react with the
support and, directly or indirectly, with the monomer. Phthalic
acid results in a covalently attached, organic, hydrophobic linker
upon which the polyaniline can readily grow. The metal Hydroxylated
surface from step 2 is immersed in the linker solution (that may
include additives such as acetonitrile to guard against premature
loss of protons) at an elevated temperature for several seconds or
minutes (details to follow). This reaction can be performed in a
dry organic solvent, such as DMSO. Suitable temperatures include
65.degree. C. for about 10 min, for example. Other temperatures and
reaction times can be used as well with acceptable results. In this
way, the surface of the non-noble metal substrate is made ready for
conductive polymer deposition. The films can be rinsed in an
organic solvent and, optionally, dried. The films can be used
immediately or stored, for example, in a container with a
desiccant. Subsequent to this, the linker can be subject to
delamination if the pH or pKA is too high and must be
preserved.
[0044] Polyaniline can be grown on either the cgg layered electrode
or the chemically modified electrode. Polyaniline grows in at least
two phases (MacDiamid), lateral surface coverage, and vertical or
connected growth. The chemical reactions that take place initially
to cover the surface of the substrate (in the preferred case the
linker layer) are thought to be different than in full growth
phase. The electrochemical potential that drives the early or
templating phase reactions can also be different. In order to
stimulate good adhesion and high surface area morphology, a CC
(constant current) or CV (cyclic voltammetry) step may be used to
establish a seed or templating layer for the conductive
polymer.
[0045] High speed growth is achieved with the benefit of a good
template or seed layer formed above. The nature of a good template
layer is characterized by good adhesion, electrical conductivity,
and a surface that promotes high surface area morphology growth. PS
(potentiostatic) or CC (constant current) growth conditions are
selected since they are nearly twice as fast as other methods, and
more preferably potentiostatic mode is preferred since the growth
is independent of the substrate surface area. Multiple
potentiostatic growth regions, at progressively higher voltages,
may be preferred to further accelerate growth. A further
explanation of this rapid phase growth is given below.
[0046] The method provides for the deposition of a high surface
area, conductive polymer, such as polyaniline, synthesized directly
onto the ribbon or substrate using a combination of electrochemical
synthesis techniques selected from: Potentiostatic (PS), Constant
Current (CC), Pulsed Potentiostatic (PP), Cyclic Voltammetry (CV).
The following is a discussion of the advantages and disadvantages
of the various methods.
[0047] PS growth is very fast and proliferates an existing
morphology but often affords a delayed initiation of growth.
Delayed initiation can lead to an oxidized surface on the metal
substrate, poor adhesion, and less desirable morphology compared to
nanowires. PS growth can direct energy toward the desired reaction
if it is well identified. CV growth stimulates many different
reactions on the surface and can help initiate favorable reactions
less well defined and missed by PS growth such as a desirable
template for high surface area polymer growth. CV also is slower in
overall growth rate when compared to PS or CC. CC growth allows for
self selecting reactions; i.e. the reactions on the surface
determine to a large extent the resulting voltage. PP is slower
than PS but creates higher instantaneous currents. This can be
useful if the voltage and time span of the pulse is varied over a
range that includes favorable reactions. In a way, this is similar
to the CV method. The overall conductive polymer growth disclosed
utilizes one or more, such as two or more of these methods to
achieve high quality seed layers and sustained rapid growth
necessary for production applications.
[0048] The conductive material is a polymer produced in situ.
Conductive polymers include polyisothianaphthene, polypyrrole,
polythiophene and polyaniline and substituted or unsubstituted
derivatives thereof. However, polypyrrole, polythiophene and
polyaniline containing polymers and their substituted and
unsubstituted derivatives are preferred for the present invention
because they are inexpensive and easily handled. Polyanilines are
most preferred. The focus herein is polyanilines. However, the
polyanilines can be substituted with the other polymers.
[0049] The disclosed method can allow for complete growth within
about 5-90 minutes, depending on amount of growth desired and
growth conditions, on an optionally moving segment of the substrate
or metal ribbon.
The Physical Growth Procedures:
[0050] Electrochemical baths motivate ions to flow within the
electrolyte solution and electrons to flow in the conducting
electrodes and connecting wires according to the voltage presented.
The voltage potentials created between the various electrodes are
fairly local due to the IR losses within the liquid and therefore
separate electrodes can be used to define different conditions of
voltage in order to control the flow of electrons and ions to and
from the electrode surfaces. The support or ribbon-like working
electrode is passed through or subjected to a first reaction zone
or bath where PS, CC, PP, or CV is applied. The support is then
optionally passed through or subjected to a second reaction zone or
bath where PS or CC is applied. Subsequent additional reaction
zones may follow. The support or working electrode is maintained in
a grounded state with relation to the auxiliary electrodes. More
than one counter or auxiliary electrodes are either exposed to the
fluid bath directly or separated by a membrane or glass frit. Each
one of the auxiliary electrodes comprises a zone in the preparation
bath. CC, CV and PP have proved useful in initiating or nucleating
growth and promoting a highly desirable nano-textured morphology
having a very high surface area. By using at least one cycle of the
CV method, a few seconds of the CC method, or at least one pulse of
PP method, an improved conductive polymer electrode growth
initiation can be made with high efficiency. Since the moving
ribbon electrode is held at or near ground potential and the
voltage potential created by the proximity of the counter
electrodes in the synthesis bath is fairly local due to the IR
characteristics of the electrolyte, areas of a single bath can be
held at different potentials, creating different growth
results.
[0051] FIG. 3 is shown as an example of growth zones created by
multiple electrode arrangements. There may be several zones created
to refine the approach. The first zone of the bath (see FIG. 3) has
at least one working electrode, at least one counter electrode and
at least one reference electrode. The working and/or counter
electrode are configured such that one can alter the voltage
potential between them. In the first zone (initiation), PS or CC or
CV or PP can be selected. In the second zone (growth), PS or CC can
be selected.
[0052] For example, when polyaniline is being produced, the voltage
potential can be changed between about -0.2 and 1.2 volts versus a
reference electrode such as Ag/AgCl (SCE or SHE can also be used).
As the ribbon passes through this zone, it is exposed to a voltage
potential that is changing, for example at a rate of at least about
5 mV/sec, preferably at least about 100 mV/sec, such as about 300
mv/sec over the range stated. Other combinations are also possible.
The surface of the ribbon substrate begins to grow polyaniline
essentially immediately at various oxidation states, while avoiding
substantial oxidation of the support metal. Thus, the invention
comprises contacting the support comprising a conductive metal
(which preferably has been previously treated either with a
cleaning, Hydroxylation and/or bifunctional agents, or
buffed/rubbed with carbon such as graphite or graphene) with a
solution comprising a monomer characterized by multiple oxidations
states and capable of producing a conductive polymer in the
presence of at least two electrodes wherein the voltage potential
or current is altered between one or more, such as two or more,
voltage states. The CV, CC or PP methods are best used in this
initiation phase. As opposed to potentiostatic growth at 0.825
volts vs. Ag/AgCl, where the polyaniline is grown in its near fully
oxidized state, the CV method for example grows polyaniline in a
full range of oxidation and reduction states which aids in the even
initiation of growth on various substrates, even on non-noble metal
supports, where usually oxidation inhibits polymer growth and
adhesion, and particularly on the prepared surfaces as mentioned
earlier. A desired nano-textured morphology can be formed more
consistently and more quickly using CV, CC and PP. The reaction is
generally completed very quickly and complete seeding of the
support can be completed in about 1 to 100 seconds, such as 1 to 60
seconds.
[0053] In one embodiment, the film can be submerged into a low pH
aqueous solution containing aniline and, for example, camphor
sulfonic acid, and subjected to a 60 second constant current pulse,
for example at 1 mA/cm.sup.2, that initiates growth of polyaniline
"buds" onto the linker end. The resulting Ni/4-APA/PANi film can
then be dried, preferably, under low oxygen conditions and
transferred to an organic electrolyte solution.
[0054] The seeded support can then be subjected to a second
reaction zone or bath where the polymer continues to grow in the
presence of an applied voltage PS or CC. The method can then be
used to proliferate the desired morphology quickly (about twice as
fast as the CV method) which is important to reducing manufacturing
cost. In this step, the voltage and/or current is maintained to
optimize polymer growth. In the example of aniline, the voltage
potential is maintained such that the aniline is fully oxidized,
preferably at about 0.8 (e.g., 0.825) volts, vs. Ag/AgCl. The step
can be potentiostatic (PS) or galvanostatic (CC).
[0055] In another embodiment, the cgg-film can be submerged in an
electrochemical cell with an electrode, e.g., a Ni counter
electrode, arranged so that it envelopes both sides of the working
electrode. A standard Ag/AgCl electrode can be used as the
reference electrode. Camphor-10-sulfonic acid .beta. or other
sulfonating compound, can be added to the growth solution and the
pH can be adjusted. A low pH is preferred, such as 1.3, and can be
readily accomplished by adding sulfuric acid. The Al/C working
electrode is placed in the growth solution between two Ni counter
electrode plates with the distance between counter electrode and
working electrode approximately 1 cm. Constant potential method can
be used to grow polyaniline films on Al/C electrodes at 0.75 V vs
Ag/AgCl reference. It is noteworthy that this growth voltage can be
50 mV lower than with growth on platinum or other bare metal
electrodes. The film can then be rinsed with or without vacuum
and/or dried.
[0056] The polymer growth step can be maintained until the desired
degree of polymer growth has been achieved. Typically, the reaction
will be maintained for at least about 1 minute, preferably at least
about 2, 3, 4, or 5 or more minutes, up to an hour or more.
[0057] The first and second growth reaction zones can be in the
same vessel or different vessels. Preferably, the zones are in the
same vessel and/or in fluid communication. It is particularly
advantageous that the zones are in the same bath using a single
solution.
[0058] The solution in the bath contains monomer and a solvent. The
solvent is preferably aqueous. The monomers are preferably a
substituted or unsubstituted aniline or pyrrole, preferably
unsubstituted aniline. The monomer is preferably added and/or
maintained in solution, preferably approaching or at saturation.
For example, aniline can be added and/or maintained in an amount of
about 0.4M.
[0059] The solution optionally contains an additional acid, such as
a strong acid including but not limited to HCl, HF, nitric acid,
oxalic acid, sulfuric acid or a sulfonic acid. Sulfonate donors,
such as sulfonic acids, including camphosulfonic acid and toluene
sulfonic acids and combinations thereof are preferred. The acids
are preferably added in a molar ratio of acid to monomer between
about 0.5:1 to 3:1. Preferably, the acid is added in an equimolar
amount to monomer.
[0060] The solution can additionally optionally contain an oxide
dopant such as an oxide of Manganese, Vanadium, Iron or Cobalt.
Such additional materials can be incorporated onto the surface of
the electrode in order to improve the capacitance and voltage range
performance of the polyaniline or other polymer. Oxides of
Manganese, Vanadium, Iron, Cobalt, etc. are commercially available
from Aldrich, Waco and the like and can be placed in suspension
during the synthesis process. Since the electrolytic performance of
the oxides will partially superimpose the performance of the
polyaniline, they can work together. The electrical conducting
capability of the polyaniline comes along with its redox
capability, and therefore there is no dead weight as in the Carbon
and/or plasticizer used in Lithium ion batteries or other
supercapacitors. This feature can improve the specific energy
calculation. It is also possible to co-synthesize oxides of e.g.
Manganese and Vanadium from their Sulphates along with the
precursors of polyaniline or polypyrrole etc. Expanded voltage
range can be thus achieved by superimposing the redox
characteristics of both types of materials. Typically, the polymer
establishes the morphology, and the oxide forms on the surfaces of
the polymer.
[0061] The reaction can be readily completed at room temperature,
e.g., about 20.degree. C., although other temperatures can be
selected as well to optimize yields, reaction times or control
polymer growth. The pH of the solution should be controlled for
consistency. The pH can be optimized for the specific reaction and
is generally less than 6.0. In one example, a pH of about 1.8 can
be used. In general, a pH between 1.2 and 1.8 can be used.
[0062] The product produced by the method can then be used directly
to make the finished product or can be subjected to further
processing. Preservation of the Oxidation state of the film is
advantageous. For example, the method can further comprise a
washing and/or drying step. Washing can be accomplished by passing
the support through a low pH water solution, electrolyte solution
or volatile organic solvent to preserve the level of film
protonation. A preferred method of preparing the film for operation
in a non-aqueous electrolyte is to use a hydrazine rinse. This
rinse occurs at room temperature and causes a stripping of all
growth related anions while providing for a fully hydrogenated,
reduced film. This strategy allows for a most efficient
transformation of the conducting polymer to switchable forms in the
non-aqueous electrolyte solution. It also is purely chemical and
does not require electrode application. By cycling the conductive
polymer in a new, say lithium salt/PC solution, a new anion system
can be established inside the polymer system. Drying can also be
achieved by controlled application of convective or radiant heat
between about 20 and 200.degree. C. and through exposure to very
low dew point dry air, or exposure to a vacuum.
[0063] The process can be preferably conducted continuously or
semi-continuously. In such an embodiment, the reactants can be
replenished into the solution continuously or semi-continuously.
Where the first and second reaction zones are in liquid
communication or in the same bath, the solution may be
homogeneous.
[0064] In another embodiment, polyaniline nanowire electrodes are
synthesized on base metals, and show stability in non-aqueous
systems. Such polyaniline nanowire electrodes, when tested in
non-aqueous electrolyte systems, demonstrate 1000 F/g of active
material (F=Farad).
Typical Growth Solution and Conditions:
[0065] 0.1 M Camphosulfonic acid; 0.1 M p-Toluenesulfonic acid;
0.45M Aniline in DI water. Growth is achieved at 25 degrees C.,
with CV between -0.2 and 1.2 Volt vs. Ag/AgCl reference at 300
mv/sec for one cycle. Then PS for about 2400 seconds at 0.75 Volt
vs. Ag/AgCl reference is performed. The working electrode can be
Titanium, Aluminum, Nickel, Stainless steel, Silver, Platinum or
Gold, graphite covered aluminum being preferred. The counter
electrode can be Nickel, Stainless Steel, Platinum or Gold,
stainless steel being preferred. The reference electrode can be
SHE, Ag/AgCl, SCE, etc, Ag/AgCl being preferred.
[0066] The methods described herein can result in nanoporous
growths having superior properties and characteristics. Conductive
polymeric nanowires can be readily grown in equal parts Aniline and
strong acids. It is found that greater stability is afforded in the
disclosed mixture: The preferred synthesis solution has one part
Aniline to the sum of the CSA and p-TSA to optimize the morphology,
redox activity and stability of the film. The selected
concentrations are 0.45M monomer/0.1 M pTSA/0.1M CSA in a water
solution. Another preferred solution contains only CSA at 0.2 M and
aniline at 0.45M (0.45 M aniline is essentially a saturated
solution). In one example, nitric acid or another strong acid can
be used to adjust the pH of the solution to about 1.3. The CV/PS
combination also provides greater adhesion which is important to
subsequent processing.
Product Characteristics:
[0067] The products formed by the process can be characterized as
having a very high surface area and are highly porous, as evidenced
by visual inspection, as below. The polymers are nanowires or
nanofibers in that the diameters of a substantial number (e.g., at
least about 50% and more preferably at least about 70%, such as at
least about 90%) of strands formed thereby are less than 1 micron,
preferably less than 500 nm and more preferably less than 200 nm.
The pores and interstitial spaces formed by the nanowires are
relatively large, in comparison to the fibers, the diameters of
which are often 4 times or more the diameter of an adjacent
fiber.
[0068] The polymers are conductive to the surface of the electrode
and possess good to excellent adhesion properties. The surface area
of the structure can be measured using nitrogen absorption, as is
known in the art and can be preferably at least about 200
m.sup.2/g, such as at least about 500 m.sup.2/g, or more preferably
at least about 1000 m.sup.2/g. Maximizing surface area improves ion
capture when the polymer is in contact with an electrolyte
solution. For example, the polyaniline electrode made in accordance
with the method above can produce about 4 volts in an organic
solvent containing 1 M lithium salts when mounted opposite a
suitable Lithium or Lithiated metal electrode such as Aluminum or
Tin. In an aqueous electrolyte solution, a 2 volt cell can be
produced with the polyaniline electrode on each side of the
cell.
[0069] FIG. 4 illustrates the nanowire and highly porous nature of
the product.
[0070] Good adhesion is defined as a film that continues to operate
without degradation over thousands of use cycles (see operating
curve below showing up to 25,000 cycles of polyaniline operation
without degradation).
[0071] Lithium ion batteries, with positive electrodes of Lithium
inserted Cobalt, Iron Phosphate, MnO.sub.2 etc. for comparison can
operate for hundreds to low thousands of cycles with up to 30
percent degradation. The TEM above is typical of the delicate
nano-structure of stabilized Polyaniline.
[0072] The products of the invention have good to excellent
resistance to cycle degradation. The products are resistant to
cyclic voltammetry testing. Resistance can be measured by immersing
the polyaniline sample in a half molar solution of Lithium salt,
such as LiBF.sub.4 in 50% PC/AN electrolyte, and cycling between
its operating voltage of -0.2 and 0.9V re. Ag/AgCl at 1000 mv/sec
scan rate for 5, 10, and 20K cycles. The products of the invention
are characterized by less than 10%, preferably less than 5%,
degradation or current loss over 2,000 (preferably over 5,000 or
10,000) cycles applying a voltage potential of 50 mV/sec, such as
100 mV/sec, or 1,000 mV/sec. The percentage degradation can be
calculated by determining the difference in the areas under the
curve before and after the stress tests are conducted.
[0073] The improved properties of the products described herein are
due in part to the homogeneous deposition of acid groups, e.g.
sulfonate groups, along the polymer structure. Thus, the invention
provides for electrodes comprising a support comprising a
conductive material and a conductive polymer, characterized by a
sulfonate concentration within 1 micron (preferably within 0.5
microns) of the support surface that is essentially the same as the
sulfonate concentration within 1 micron (preferably within 0.5
microns) of the electrode surface. "Essentially the same"
concentration is intended to mean herein that the sulfonate to
monomer ratio at the two loci is within about 50%. Thus, if the
sulfonate to monomer molar ratio at the surface is about 0.5:1,
then the sulfonate to monomer molar ratio at the support is between
0.25:1 to 1:1.5. In another preferred embodiment, the sulfonate to
monomer ratio at the surface and at the support (within 1 micron,
preferably within 0.5 microns) are both approximately 1:1.
[0074] In order to preserve the level of dopant anions in the film
electrode during transfer out of the aqueous growth solution and
into the organic electrolyte, the film is left in the fully charged
state. This is characterized by a dark green color, and represents
full oxidation. Although the large anions leave the electrostatic
attraction of the polymer during discharge, they are physically
trapped unless they are inadvertently washed away during the
transfer process. It was found that a fully charged film would
provide greatly enhanced cycling endurance once transferred into
organic electrolyte. Another and most preferred strategy is to
strip the anions from the film while preserving the level of
hydrogenation by using a 5% Hydrazine rinse. In this condition, the
conductive polymer is stable chemically in the reduced state and is
white or cream in color. A high percentage of the repeat units are
now available to be made electrically active in the new anion
system provided by non-aqueous electrolyte solution.
Manufacturing Improvements:
[0075] During the polyaniline growth in the aqueous growth bath,
the nanowire structure is fragile and subject to damage. In order
to minimize the risk of damage to the growing film, care should be
taken in the handling of the film as it travels through the growth
bath. Potential damage to the film can come from contact with the
guiding/turning rollers, contact with the electrode plates, induced
surface shear from fluid flow and normal (pressure) forces on the
film. The mechanical design of the growth tank preferably accounts
for all these possible modes of film damage over the entire
operating range. During normal operation, the film can be traveling
at a speed of 10-20 cm/sec, and the design can also handle zero
speed conditions and start up/speed up and stop/slow down
transitions without damage to the films. To accommodate these
design requirements, a number of design approaches are
preferred.
[0076] First, guide rollers or turning bars can be used to change
the direction of the film in the growth bath. For a single layer
growth, either horizontal or vertical, the guide rollers are used
to direct the film from the external source, down into the growth
medium, through the electrode plate path, and then back out to the
film exit. A basic vertical arrangement uses at least one guide
roller in the growth medium, and a basic horizontal arrangement
uses at least two guide rollers in the growth medium. In both
cases, vertical and horizontal, the external routing of the film
into and out of the tank will also typically use guide rollers.
[0077] Typical film tensions will be in the range of 2-10 N (0.4 to
2.2 lbs) and this tension can be maintained at all times during the
manufacturing process, and all film speeds from stationary to
maximum velocity. The film tension can be maintained by components
external to the growth tank such as core shafts, pneumatic shaft
brakes and clutches, tension sensing rollers and closed loop
tension controllers. Nimcor and Montalvo supply such equipment.
During normal processing speeds, the film can be pulled into the
tank, around the guide rollers, through the electrode field plates
and back out of the tank. While the tank design illustrated herein
has been for horizontal processing of the film between horizontal
field plates, vertical designs can be used as well.
[0078] While the film is pulled around the guide rollers (FIG. 19),
there is risk of damage to the film surface through a skidding
contact (differential motion) of the film against the roller
surface. There are several ways to minimize the risk of this type
of damage. Even though the substrate and growth structures are very
thin, when the film is pulled around the roller, the neutral axis
for bending will exist close to the center of the film and the
inner surface (surface facing the roller) will compress and the
outer surface will be placed in tension. This compression as the
film is pulled from a straight condition into a curved state will
cause some relative motion of the film surface against the roller
surface. This motion can be minimized by making the roller diameter
very large compared to the film thickness. The guide rollers are
typically in the 75-100 mm diameter range to help minimize this
relative motion of the film surface compared to the roller surface
and neutral film axis. A more important source of differential
motion is caused by differences in motion between the film and the
surface speed of the roller. The rotation of the roller can be
driven by the motion of the film through shear stress, or the
roller can be driven by an external source such as a motor. In the
case where the roller is driven by an external motor, a control and
drive system can be built to match the roller velocity to the film
velocity, but small variations due to control errors will always be
present, especially during periods of velocity change were the film
is either speeding up or slowing down. Even if the roller and film
are driven by the same motor drive, rotational inertia and drive
backlash conditions can cause momentary velocity differentials. In
the case where the film drives the roller speed, there can be an
induced shear force on the film surface to transfer the required
torque to overcome the bearing and fluid drag on the roller. During
a velocity change, the shear force between the film and the roller
will also have to account for overcoming the rotational inertia of
the roller. Very slow acceleration and deceleration can be used
minimize this induced inertial drag on the film surface, but this
leads to long transition times which can make controlling the
growth process between the field plates very difficult.
[0079] A better solution to minimizing the surface shear on the
film and to eliminate the risk of a differential skidding motion is
to create a fluid layer between the film and the roller surface.
FIG. 19. This fluid layer can support the film, preventing contact
to the roller surface and due to the low viscosity of the aqueous
growth solution, velocity differentials between the film and roller
will create only very small shear forces on the film surface. At
running speeds, it may be possible to use entrained fluid on the
roller to produce this fluid layer, but for protection over the
entire operating film speed range, the fluid film is maintained
using an external mechanism. By pumping fluid through the roller
surface from the roller inner diameter to the outside, a fluid film
can be maintained, even at zero film speed. Also, by doing this,
the rate of pumping can be controlled to accommodate varying film
tensions.
[0080] For a given line tension T, the preferred pressure to
produce a fluid gap can be estimated using the following
equation:
2T=.intg..sub.o.sup..pi.P(.theta.)wrsin(.theta.)d.theta.
where P is the local gap pressure, w is the film depth into the
page and r is the roller radius. For a simple initial calculation,
one can assume that the pressure is constant around the
circumference and the equation becomes:
2 T = 2 P w r ##EQU00001## or ##EQU00001.2## P ~ T w r .
##EQU00001.3##
[0081] Thus, for a 10 N tension with a 20 cm wide film on a 75 mm
diameter roller, the preferred pressure would approximately be 667
Pa (0.1 psi). These low pressures are very easy to establish and
minimize the differential pressure across the film as well.
[0082] A system has been designed that incorporates a perforated
roller onto a driven hollow shaft. Fluid is pumped into the hollow
shaft through rotary unions. This fluid then enters the inner
diameter of the roller and exists through the holes in the roller
shaft. An isometric view of this roller is shown in FIG. 20. In a
preferred embodiment, the fluid is a reaction medium.
[0083] The guide rollers described herein can be used in other
methods for transporting fragile or thin sheets of material through
liquid or fluid solutions.
[0084] The electrochemical growth tank can employ one or more
potential electrodes. Preferably, the process employs plates with
multiple potentials for initiating, growing and conditioning the
polyaniline structure. Use of multi-potential electrodes will
facilitate a continuous roll fed foil system. (FIG. 21). Further,
the electrodes can be configured serially and/or stacked.
Preferably, the process employs rows of horizontal electrodes
stacked in vertical series. The low shear roller structures allow
the foil or film to be wrapped back and forth within a single tank
and reduce the total line length significantly. Each electrode in a
horizontal set can be at the same or different potential and each
vertical series can also have the same or multiple potentials.
[0085] In a batch process, there is a single electrode to which
different potentials or current conditions can be applied in order
to obtain the desired result. In this case, the operating
conditions can change with time and can include constant voltage,
constant current and constant power operation. With a continuous
foil process, time varying conditions on a single electrode or set
of electrodes is not optimal. Instead, the system preferably uses a
series of electrodes at different conditions that the foil is
pulled past. By changing the conditions from one plate to the next,
an effective time varying condition can be applied to any fixed
point on the film.
[0086] FIG. 21 shows a system with layers of electrodes, in which
there are three separate sets of electrode plates per layer. Each
set of plates consists of an upper and lower plate such that both
sides of the film is exposed to the same potential. The electrodes
are preferably electrically isolated from each other. They can all
be connected together externally in order to be run at the same
operating condition, or each set can be run at an independent
operating condition. FIG. 21 illustrates a device where a film can
be fed past the input roller, around the guide roller between a
first set of electrodes, between a second and third set of
electrodes configured in a horizontal series, exit the electrode
set under and around a second guide roller and back through a set
of three serially configured sets of electrodes. These sets of
electrodes are configured vertically above the first series of
three sets of electrodes. The film can then exit the assembly to
under and around a third guide roller and back into a third series
of three sets of electrodes and so on. Five layers of three sets of
electrodes is shown. FIG. 21 also illustrates the tank and frame
for the electrodes.
[0087] The length of each electrode set can be consistent between
sets or can vary from set to set. For a given foil speed, a series
of short electrodes sets spaced closed together can be used to
produce a rapidly time varying voltage or current condition. Long
electrode sets or multiple sets that are electrically connected can
be used to produce time constant voltage or current conditions. The
size of the gap between the electrodes in a set can be varied
depending on the film speed, size and length of a single layer. For
a given film tension, the film cannot be allow to contact the
electrodes, so as the length of the single layer increases or as
the density of the tank fluid decreases or as the thickness of the
film increases, the size of the electrode gap preferably increases
to accommodate increased dropping of the film. In general, a 10 mm
gap will handle all possible film/fluid/tension operating
conditions. Making the gap too large will negatively impact the
size of the overall system as well as increase potential electrode
edge issues in terms of a degraded electric field in the growth
fluid.
[0088] The gap between successive sets of electrodes is dependent
on the fluid properties (resistance/length) and the electrode
potentials. As the conductivity of the fluid increases, or as the
electrostatic potential between successive sets increases, the
minimum allowed gap between the sets preferably increases to
prevent excessive leakage effects from one set of electrodes to
another. Even for conditions where there is a single potential to
be applied to the electrodes, the system is preferably split up to
a series of independent electrode sets that are independently
controlled. As concentrations of chemicals and salts vary, the
effective potential of an electrode set compared to the local film
can vary. In order to keep the local electrode set at the correct
potential, a reference electrode may be used to account for varying
fluid conditions. Especially in cases of very long layers at a
single potential, the local reference can be used to keep a tight
tolerance on the effective electrical potential and the system can
be broken down into a series of independent electrode sets that
will run at similar, but not exactly the same, potentials.
[0089] Another factor that may benefit from the separation of the
electrodes into multiple sets is the grounding requirements of the
conductive film. As the film travels through the tank and is either
plated, or has a polyaniline structure grown on it, an electrical
current is passed into or out of the foil, depending on the
operating conditions. This accumulated current must complete the
driving circuit and be conducted to the opposite power supply
terminal from the electrode set. This requires a regular
mechanical/electrical connection from the film to the power supply
terminal. As the current flows through the foil to this common
collector point, the current flow will induce a voltage potential
in the film. If the distance along the film between successive
collector points is too high, then the induced voltage drop can be
high enough to start to affect the plating or growth process. For
example, if you have a 10 micron thick aluminum foil (1145 alloy,
for example), that is being subject to a constant current state of
1 mA/cm.sup.2 per side with a total length between grounding points
of 1 m, then the maximum potential difference along the film will
be approximately 7 mV. If the current or the length is doubled,
then the maximum potential difference along the film will be 14 mV.
If the process requires a voltage potential variation limit of 10
mV, then the first instance may be tolerated, but the second would
require breaking the electrodes into multiple sets with film
grounding in between the sets.
[0090] Another benefit to breaking the electrodes into sets is to
allow for multi layer tank structures. For a required residence
time of a foil in a plating or growth potential, the required
wetted length of the electrodes is a given. For example, a required
residence time of 2 minutes for a film traveling 10 cm/sec will
require a total wetted electrode length of 12 m. For a single layer
approach, this would require a tank of 12+ meters in length. By
breaking the electrodes into sets and then stacking multiple layers
of sets in a single tank, the overall tank length can be reduced
significantly. If the previous example is broken into 4 layers,
then the total layer length would be 3 meters and the tank might be
3.5-4 meters total. Due to the very delicate condition of the
polyaniline in the aqueous state during the growth process, this
can be facilitated by using the low shear guide rollers/turning
bars described above.
[0091] Two electrodes composed of polymer synthesized as described
above can be mounted opposite each other, separated by electrolyte
and a semi-permeable membrane in order to make an electrolytic
super-capacitor. For example, the ribbon electrode can be cut into
shapes facilitating assembly. One example is the use of laser
cutting that includes the forming of an outer shape and the
electrode contacting hole or spacer. Material including the punched
sprocket holes can be retained or discarded after trimming. A
vacuum handling device can be used limiting contact to small
portions of the electrode sheet.
[0092] An example of an assembly is enclosed where sheets of
electrodes are separated by semi-permeable membranes. The sheets
are wetted with an electrolyte mixture, for example, by air-free
atomization or submersing the electrodes into an electrolyte
solution. Air-free atomization can avoid the use of vacuum filling
techniques that can harm the nanostructures. The electrolyte easily
wets through the semi-permeable membrane and into the polymer
electrode, controlling excess fluid.
[0093] In the case of a product with opposing polyaniline
electrodes, aqueous electrolytes can be used. Such products can
operate between 0 and 2 volts, for example. The electrolyte
solution can be an ionic liquid, e.g., a room temperature ionic
liquid such as 1-butyl-3-methylimidazolium chloride, sulfuric acid,
potassium hydroxide, sodium hydroxide, propylene carbonate,
dimethoxy ethanol, diethyl carbonate or acetonitrile. As further
examples, the liquid may include LiClO.sub.4, NaClO.sub.4,
LiAsF.sub.6, LiBF.sub.4 or quaternary phosphonium salts.
[0094] In the case of higher voltage operation (e.g., 4 volts)
using polyaniline and Li/Al requiring high breakdown field
operation, electrolyte solutions such as propylene or ethylene
carbonate/dimethoxyethane (PC/DME, such as 50/50 PC/DME) and
Lithium salts such as LiBF.sub.4 can be used. Additionally, a large
negative electropotential material can be selected as the opposing
electrode. As is typical for a Lithium ion battery, a Lithium based
negative electrode can be used. Lithium ions can be inserted into
Carbon or Graphite which results in an electrode of about -3 Volt.
Lithium ion batteries suffer from overheating and sometimes
explosion due to the use of over -3 volts conditions during
charging. Since -3 Volts represents a decomposition Voltage for
Propylene Carbonate as well as other organic electrolytes, it is
desirable to find a safer negative electrode if possible.
[0095] The inventors discovered that safer negative electrodes can
be made by inserting lithium into an aluminum electrode. In this
embodiment, aluminum, and alloys thereof, can be selected as a
Lithium ion insertion material since its alloyed electro potential
is about -2.7 Volts, well within the "safe organic electrolyte
operating window." The aluminum is preferably a magnesium
containing alloy. The alloy is lithiated by flowing a reducing
current (e.g. 5 ma/cm.sup.2) in a preferably moisture free organic
electrolyte containing a Lithium salt such as LiSO4, LiBF.sub.4 or
LiClO.sub.4. The lithium salt is preferably maintained in a
solution of at least about 0.5M, preferably about 1 Molar solution.
An organic solution, such as PC/DME as above can be used. The dried
Aluminum can be lithiated as described above or by physical
application with an air insensitive Lithium compound such as SLMP
(Stabilized Lithium Metal Powder) from FMC Corp. In the latter
case, the Lithium is alloyed into the Aluminum by rolling or
contacting with electrolyte solvents. As specified by Melendres in
U.S. Pat. No. 4,130,500, which is incorporated herein by reference,
Magnesium can help physically stabilize Aluminum film during the
swelling and shrinking phases of Lithium insertion and removal,
respectively.
[0096] Specifically, an alloy composition of about 1, preferably 2
atom percent magnesium to 20 atom percent magnesium and aluminum
can be used. At least 3 atom percent magnesium can assist in
maintaining structural integrity during electrical discharge when
the lithium atoms migrate to the electrolyte. Examples of specific
Aluminum alloys include 5083 and 5052. Since the magnesium does not
enter into the cell reaction, it appears to serve in interstitial
or substitutional solid solution as a bonding material for
maintaining a matrix structure into which the lithium can be
repeatedly charged. Magnesium concentrations are preferably less
than 20 atom percent. Specifically, an alloy composition of about 2
atom percent magnesium to 20 atom percent magnesium and aluminum
can be used. At least 3 atom percent magnesium can assist in
maintaining structural integrity during electrical discharge when
the lithium atoms migrate to the electrolyte. Examples of specific
Aluminum alloys include 5083 and 5052. Since the magnesium does not
enter into the cell reaction, it appears to serve in interstitial
or substitutional solid solution as a bonding material for
maintaining a matrix structure into which the lithium can be
repeatedly charged. Magnesium concentrations are preferably less
than 20 atom percent. As noted in the literature (e.g. Schleich et
al., J. Power Sources, 2001) the thinnest aluminum samples were
less damaged in electrochemical tests. The micro-porosity created
as a result of the etch (described elsewhere) yields a similar
result by improving the ionic access to the film which in turn
improves the cycling capability of the Li/Al alloy. In the etched
aluminum film, the thinner walls (compared to a monolithic aluminum
sheet) allows the aluminum to expand and contract with less damage.
The source of the improved performance is thought to be a reduced
concentration gradient within the electrode.
[0097] Lithium is added to the aluminum-magnesium alloy in
concentrations of about 5 to 50 atom percent. Lithium is preferably
added to the magnesium-aluminum alloy electrochemically. This can
be performed as the initial charge within an assembled cell having
sufficient reaction product, e.g. a lithium sat or chalcogenide, to
provide lithium into the negative electrode composition. Lithium
and aluminum can also be mixed in a powder form and rolled into a
foil, and annealed at 500 degrees C. for use.
[0098] Platinum can be used as the counter electrode (Ag/AgBF4 in
PC/DME is can be used as the reference electrode in the organic
electrolyte). An excess of Lithium is inserted to the Aluminum to
achieve a -2.7 volt electrode (usually between 2-50% Molar
content). The amount can be optimized for the desired final
current.
[0099] Although Lithium alloyed or amalgamated Aluminum is less
sensitive to moisture that Lithium metal, low moisture conditions
such as -40.degree. C. dew point air or Nitrogen gas are preferably
used after the Lithiation process. In order to increase the surface
interaction sites, the Aluminum can be electrochemically roughened
or etched in 1 M HCl, 5% ethylene glycol at room temperature with
an oxidizing current of at 0.02 A/cm.sup.2 to form a very high
specific surface area electrode which increases its current
capability. Alternatively, the etch is between 10 and 30 seconds at
80.degree. C. Three minutes at room temperature is most preferred
in order to provide the necessary surface area without weakening
the substrate. The substrate is now dried at elevated temperatures
of 80 to 120 degrees C. in dry air. In this way, a hybrid
super-capacitor can be made with up to a 4 Volt or so operation
(e.g. -2.7 volt Li/AL plus 0.9 volt Polyaniline=3.6 volt operation)
as illustrated below (FIG. 5) for a representative electrode
system.
[0100] In another embodiment, a high surface area lithium-aluminum
electrode is prepared by electrochemically etching an aluminum
alloy employing an HCl-ethylene glycol solution. Examples of
aluminum alloys include, but are not limited to, the 5xxx series,
containing 0.3-5.0% magnesium.
[0101] The ribbon electrodes are advanced continuously through the
pre-treatment, growth, etch or Lithiation baths before going
through the drying tunnel. The drying tunnel simply applies a
ramped heating in air at between 20.degree. C. to 200.degree. C.
gradually, to eliminate water from the material. Alternately,
drying can take place by immersion in organic solvents, and then
dried as above. The simple, rinse free drying leaves behind a
certain beneficial level of acid salts as a further source of
dopant to the film.
[0102] Improvements have also been made to the drying step of the
process. Drying the incoming material (+ substrate, - substrate and
film layer) is important to minimize the introduction of moisture
into the final assembly area and into the final package. Since both
the polyaniline structure and the negative substrate are treated in
aqueous solutions, there is a significant amount of moisture that
has to be driven off without damaging the films. In order to do
this, the materials can be passed through an infrared (IR) heating
oven with a preferably counterflowing dry air/gas stream. Cross
flowing streams can also be used. Use of long IR (2-10 micron)
heaters helps to prevent damage to the grown polyaniline structure
which is sensitive to degradation from visible light. Also for
drying the etched aluminum substrate, the use of long wavelength IR
will focus the energy on the resident water rather than the
aluminum surface, which is highly reflective of long IR. The
counterflowing dry air/gas helps to prevent the evaporated water
vapor from entering the final assembly area and increases the
drying rate compared to IR heating in ambient conditions. Reducing
the gap size around the foil helps minimize the required volume of
dry air/gas and results in a higher counterflow velocity for a
given flow rate of air.
[0103] In one embodiment, a pair of IR lamps are mounted with their
emitting surfaces facing each other across a small gap (10-20 mm).
The lamps are mounted into a frame assembly that has aluminum
panels mounted on the outside to create a sealed volume with either
one or two entry gaps for incoming/exiting foil material. If a
single gap on one side of the tunnel is used, then discrete
sections of material can be inserted, dried and then removed. If a
gap on both sides is used, then film can be passed continuously
through the tunnel. Each IR heater has an embedded thermocouple
sensor and its own PID temperature controller. The operating
temperature of each heater can be independently set. There is a
port on the side of the tunnel to allow for dry air/gas entry.
[0104] This small tunnel is sufficient for lab tests and basic
development, but does not provide true counterflowing dry air/gas
conditions. For the production line, the drying tunnel is the
interface between ambient operating conditions and a dry air
environment. The dry air environment can be kept at a higher
pressure than ambient to prevent humid air and moisture from
leaking into the dry air manufacturing volume. A narrow gap IR
drying tunnel can have multiple heaters on the top and bottom
(e.g., 2, 3 or more on each top and bottom) in an arrangement that
can be optionally hinged open for loading the film into the tunnel
and for access to clean the IR heater surfaces for maintenance. By
putting multiple heaters along the length of the tunnel, the
heaters can be set at different temperatures if a controlled
ramping of the film temperature is needed. In the case of a wet
polyaniline film, rapid heating of the water and the substrate may
lead to damage of the film, including up to the ablation of the
polyaniline from the substrate surface. Multiple heaters allow for
the controlled ramping of the energy input into the films to help
prevent this. Between successive heaters, a highly reflective
surface is used to reflect/reradiate any incident energy. These
surfaces can typically be polished aluminum or stainless steel
sheets, e.g., 1-2 mm thick. The position of the IR heaters within a
module is flexible and can be arranged to optimize the drying of
the different films. If an unplated etched aluminum film needs to
be dried, the heaters can be moved as far away from the film
entrance as possible. This exposes the film surface to a period of
dry counterflowing air before exposure to IR energy. This initial
area of dry air will drive off some of the resident water. Since
the oxidation rate of exposed aluminum is highly affected by the
presence of steam vapor, it may be advantageous to drive off as
much water from the film as possible before starting to drive up
the surface temperature of the aluminum. For drying the polyaniline
film, a gentle ramp up to temperature may be needed and multiple
close spaced IR heaters with each successive heater having a
slightly higher operating temperature may be optimal. Gap sizing is
important to keep to a minimum to reduce the volume of
counterflowing dry air and to raise the air velocity relative to
the film. As the air velocity increases, the coefficient of
convective heat transfer and mass transfer will increase, helping
the drying process. Also as the dry air flows along the drying
tunnel, it will pick up thermal energy from the emitters,
reflectors and side walls. This increase in air temperature will
help to increase the drying rate. FIG. 22 illustrates such a
tunnel.
[0105] Both dry air and inert gases can be used for the drying
process. In some cases where the film may be especially subject to
oxidation or degradation from exposure to air, an inert gas such as
N.sub.2 or Argon may be used to maintain a stable gas layer around
the film. Minimizing the gap size will help to reduce the required
gas volume.
[0106] The modular drying tunnel may also be stacked in series to
develop a more flexible drying station for various conditions. The
modular drying system consists of series of drying tunnels in
series with a guide roller assembly at the front and rear of the
system. The tunnels can be stacked in direct contact to maintain an
effective gas tunnel for the counterflowing dry air. The guide
rollers are used to ensure that the film remains in the middle of
the air gap in the tunnel. If required, especially on long tunnels,
guide roller assemblies can be place in the middle of the drying
tunnel system. The final set of guide rollers at the exit of the
tunnel are enclosed within a dry air enclosure that will mate to
the final manufacturing dry air volume. This volume will be kept at
a higher than ambient pressure and this pressure will induce the
counterflowing air stream through the drying tunnel. If a set of
intermediate guide rollers is used, it also has to be placed within
a dry air enclosure.
[0107] After drying, the ribbon electrode is cut into shapes
facilitating assembly. Examples include the use of laser and
mechanical cutting that includes the forming of the outer shape and
the electrode contacting hole or spacer. Material including the
punched sprocket holes can be discarded after trimming. A vacuum
handling device touches only small portions of the electrode sheet.
An example of an assembly is enclosed where sheets of electrodes
are separated by semi-permeable membranes. The sheets are wetted
with electrolyte mixture during assembly in this case. An air free
atomization is used to wet the film sheets during construction to
avoid the usual vacuum filling techniques that can harm the
nano-structures. The electrolyte easily wets through the
semi-permeable sheet into the polymer electrode, and excess fluid
can be controlled. In the case of a low voltage product with
opposing polyaniline electrodes, aqueous electrolytes can be used.
In the case of higher voltage operation using polyaniline and Li/Al
requiring high breakdown field operation, electrolyte solutions
such as PC/DME and Lithium salts such as LiBF.sub.4 are used.
[0108] The Al substrate enters the tank on the left hand side and
is routed to the bottom of the tank, and then routed back and forth
over several layers while being pulled between multiple isolated
electrodes. The gap between upper and lower electrodes is shown
here to be 9-10 mm and the electrodes have ports for inducing flow
of the aqueous solution along the substrate path. This fluid flow
supports the film during growth, preventing contact with the
electrodes and eliminating shear forces on the nanowire structure
during growth. Similarly, shear forces on the nanowire structure
where the substrate travels over the guide rollers can be
eliminated by driving the rollers at line speed to avoid any
velocity differential, and by pumping fluid through the hollow
perforated rollers to maintain a fluid film between the rollers and
the substrate. The substrate tension will be approximately 8 N and
will require local fluid pressures of only 1.6 kPa (0.24 psi) to
maintain a fluid gap between the roller surfaces and the substrate.
This fluid pressure is easy to obtain through entrained fluid on
the driven rollers and by pumping the aqueous solution through the
perforated rollers. These steps are specifically done to minimize
all shear and non-uniform normal stresses on the nanowire structure
during growth.
[0109] Once the film with the nanowire structure leaves the growth
tank, it will be pulled through a drying tunnel that uses
incremental medium wavelength IR heaters and heated dry air
(-40.degree. C. dewpoint) to remove the moisture. Once dried, the
nanowire structure is more robust and easier to handle without
damage. The remaining steps and handling of the substrates will be
through use of vacuum belts and vacuum grippers. Due to the very
light weight, low tension forces and the large surface areas, very
low levels of vacuum (<1.3 kPa, <0.2 psi) can be used for
handling purposes. These very low, uniform pressures significantly
reduce the risk of nanowire damage.
[0110] A semi-permeable membrane can be used to electrically
isolate the two electrodes but allow ion transport (e.g. Li+ and
BF.sub.4--) between the electrodes. Since the synthesized
polyaniline formed from this method is brush or branch like, a
separator may have periodic pillars or spacers incorporated to
protect the film, as shown in FIGS. 7A and 7B.
[0111] The pillars (1-2% of the total area) incorporated onto the
surface of the separator membrane crush into the film (not less
than 4 microns thick) and establish support without sacrificing the
remaining 98% or so nanostructures and their redox providing
surface areas. In this way, force can be used to mount the various
layers of the multilayered device without crushing or reducing the
available surface area of the polymer nano-structured films. The
lost area due to the spacer arrangement is about 1% and the spacers
can be crossed lines, round or square shapes or other shapes that
provide stand-off capability. The semi-permeable spacer or
separator can be made of polypropylene, Teflon, poly carbonate,
cellulose based or other materials. Not to limit the approach, but
these can be pressed, etched, machined, ablated or molded. The
materials can be purchased from e.g. Dupont, Celgard, LLC, etc. The
films are then pressed under heat to form the pillar structures,
and then expanded in the usual ways to provide the ionic transport
capability. Another method for providing the pillars or supports is
by printing dots or bumps of epoxy or other suitable material to
the permeable membrane by using micro-gravure or direct ink jet
methods. The permeable membrane may be treated with surfactant and
other additives as known in the art.
[0112] Typical pressing conditions are 2000 psi at 180.degree. C.
and depend on the melting point of the exact material. Since the
pillars only represent 1% of the area, even 25 micron pillars can
be produced without losing even 1 micron of film thickness. The
features are formed in a roller or calendaring technique. A second
technique is the use of a salt electrolyte/plasticizer/filler to
mold a working separator with pillars or spacers out of electrolyte
containing compounds known in the art. A third way is to provide a
separate spacer device combined with a semipermeable membrane,
separately or attached. Typical compounds are EC, PC, DME, PVDF,
TFE and the salts are typically LiClO.sub.4, LiBF.sub.4 etc.
[0113] Alternating layers of cathodes and anodes are stack on top
of each other with spacer layers that are electrically insulating
but ionically conductive between each successive anode and cathode
layers. Alignment is such that all the anode layers extend out of
one side of the stack and all the cathode layers extend out of a
different side of the stack. FIG. 8B shows this alternating stack
arrangement with the cathode and anode layers extending out of
opposite sides of a stack. The spacer layers are slightly wider
than the anodes and cathodes and prevent the edges of the stacked
sheets from coming into contact with each other. The individual
anode layers are welded or mechanically staked together and then
joined by the same means to a contactor bar. The cathode layers are
similarly joined together and to a cathode contactor bar. This
entire stacked assembly is then placed into a sealed package or
case with the ends of the contactor bars passing through the
package for external connection to other components. This type of
construction allows for variable layer counts for different battery
capacities and applications. This type of construction also
minimizes ohmic losses during charge and discharge. A final package
is shown in FIG. 8C. The connector bars for the anode and cathode
layers can pass through the final package or case on either one
side each for lower power systems, or on both sides for higher
power/higher current applications. The connector bars can be
welded, riveted or staked to the battery layers. The external
sections of the connector bars can be easily connected to external
circuit components such as electrical connectors or cables.
[0114] Stacks of alternating electrode and spacer layers are
produced, connecting to negative and positive posts or terminals
(symmetrical device shown in illustration FIG. 8A) alternately.
Notches or holes on alternating sheets of electrode are cut out to
prevent contact to the opposite electrode/alignment pins.
[0115] In another embodiment, a low-cost supercapacitor
(ultracapacitor) or battery has extremely high specific energy,
energy density, specific power, power density, capacitance, and/or
electrode surface area.
[0116] In a further embodiment, the ultracapacitor or battery
consists of a layered stack of alternating positive and negative
electrodes with separation membranes between the electrodes. The
positive electrodes are made with organic polymer nanowires that
are chemically synthesized onto a base metal substrate through a
linking strategy. The positive electrodes are grown with an
electrochemical method that can be automated inexpensively. The
negative electrodes are composed of a high surface area
aluminum-lithium amalgam and are easily produced in a continuous
electrochemical process.
[0117] In yet another embodiment, the ultracapacitor requires the
combination of high energy density (high surface morphology),
successful synthesis on commodity metals, and successful transfer
to organic electrolytes.
[0118] Final assembly of the cells preferably takes place in a dry
environment to prevent reaction of the different materials with
water. Typically, the environment must be at a -40.degree. C.
dewpoint or lower. This type of environment is usually generated
used large desiccant air drying systems. By automating the assembly
process and keeping the required dry air volume in the
manufacturing area to a minimum, the size and operating cost of a
desiccant system can be minimized. Air drying units can be very
large sources of power consumption due to their use of heat to
regenerate the desiccant beds and due to the large blowers required
to maintain air flow. Instead of conducting the final assembly
process in a room environment where the volume is large and where
people are required to operate, automating the process and keeping
the volume to an absolute minimum will save in equipment cost and
operating costs.
[0119] The equipment is designed around a robotic assembly station
with preferably three incoming material feed lines and a secondary
assembly chamber. The material feed lines consist of a set of
process equipment that can include cutting stations, material feed
drives, guiding stations and, in some cases, a material process
tank that contains a non-aqueous solution. Each line can have a
counterflow drying tunnel to dry the incoming material and prevent
moisture ingress through the incoming material.
[0120] By making the enclosures just large enough to contain the
equipment and with access panels with dry box gloves, the volume of
dry air required can be a fraction of that required in a
traditional assembly space while still allowing for easy access to
various points in the manufacturing equipment. The dry air
enclosure includes an aluminum frame structure made from extruded
profiles with acrylic and/or polycarbonate panels mounted within
the frame. Each panel can be removed for access to the internal
machine components and also a set of dry box gloves (long sealed
gloves) can be installed in any panel where frequent access or
intervention may be required. The dry box gloves allow for operator
access while maintaining an air tight and water vapor tight seal.
The process equipment is preferably close enough to an enclosure
panel to allow for easy operator access. This prevents the need for
a person operating within the dry air volume and reduces the
moisture load on the system. An airlock is built onto the secondary
assembly volume to allow for the insertion and removal of material
from the dry air volume. Total enclosed volume for this system can
be about 24 m.sup.3 (850 ft.sup.3). This is a quarter of the volume
of a 6 m by 6 m.times.2.4 m (20'.times.20'.times.8') assembly room
and there is no burden on the system to remove the moisture load
from assembly personnel.
EXAMPLES
[0121] The materials and processes of the present invention will be
better understood in connection with the following examples, which
are intended as an illustration only and not limiting of the scope
of the invention.
Example 1
Preparation of PANi Electrode: Ni/4-APA/PANi
[0122] Polyaniline is anchored to a metal (M) surface using an
aniline derivative as a linking agent (L) to create a M/L/PANi
electrode. The nickel surface is first cleaned by chemical or
polishing techniques. The surface is then hydroxylated using three
cyclic voltammetry sweeps between 0-0.5 V (vs. Ag/AgCl) at 20
mV/sec in a 2M aqueous KOH solution to form Ni--OH moieties. The
film is rinsed and a solution containing a linker such as
4-aminophthalic acid (4-APA, CAS#5434-21-9) is allowed to react
with the Ni--OH surface by heating in DMSO solution at 65.degree.
C. for 10 min. The application of heat in a "dry" organic solvent
creates a covalent bond between the Ni--OH surface and the
carboxylic acid end of the 4-APA linker, and occurs with the
elimination of water. The resulting Ni-4-APA films are rinsed with
fresh DMSO, cooled to room temperature and either used immediately
or stored in a container containing a desiccant. We note that once
the "organic" "hydrophobic" linker is covalently attached to the
nickel surface, the coated electrode is stabilized and can be used
in aqueous solutions to grow polyaniline fibers utilizing the NH2
group on the end opposite of the 4-APA linker. Polyaniline is then
grown onto the Ni/4-APA/films using a two step procedure which
involves submerging the films into an low pH aqueous solution
containing polyaniline and other chemicals such as camphor sulfonic
acid, and immediately subjected the film to a 60 second constant
current pulse, @ 1 mA/cm.sup.2, that initiates growth of
polyaniline "buds" onto the linker end. We note that this step
appears to be critically important to producing high surface area
growth. The resulting Ni/4-APA/PANi film is then dried under low
oxygen conditions and transferred to an organic electrolyte
solution. A CV of the resulting electrode is shown in FIG. 16.
Example 2
Preparation of PANi Electrode: Al/C/PANi
[0123] Polyaniline is anchored to a metal (M) surface using a
carbon-based (carbon, graphite, or graphene=cgg) linking agent (L)
to create a M/L/PANi electrode. The Al surface is first
cleaned/etched by chemical techniques. The graphite (TIMREX High
Surface Graphite, HSAG 300) is deposited on the Al by orbital
sander (RYOBI Model # P400). Graphite deposition is carried out for
approximately 2 seconds per side of Al with the rate of 11000
orbits per min. Polyaniline is electropolymerized on Al/C electrode
using a 3-electrode configuration in a single compartment
electrochemical cell with a Ni counter electrode arranged so that
it enveloped both sides of the working electrode. A standard
Ag/AgCl electrode is used as the reference electrode. The Ni
counter electrodes are cleaned by polishing and chemical techniques
and immersed in the growth solution containing 0.45 M aniline
(Sigma-Aldrich, CAS#62-53-3), 0.2 M camphor-10-sulfonic acid .beta.
(Sigma-Aldrich, CAS#5872-08-2) and pH is adjusted to 1.3 by adding
H2SO4. The Al/C working electrode is placed in the growth solution
between two Ni counter electrode plates with the distance between
counter electrode and working electrode approximately 1 cm.
Constant potential method is used to grow polyaniline films on Al/C
electrodes at 0.75 V vs Ag/AgCl reference (MACCOR, model # series
4300). It is noteworthy that this growth voltage is 50 mV lower
than with growth on platinum or other bare metal electrodes. FIGS.
17A and 17B show a typical CV in aqueous and non-aqueous solutions,
respectively. The film is then rinsed in 5% hydrazine
(Sigma-Aldrich, CAS #302-01-2) solution for 5 min with or without
vacuum. The resulting film is then dried by transferring it to an
Infra Red drying chamber (300.degree. C.) with or without N.sub.2
flow for 5 min. The dried polyaniline film is then transferred into
0.5M LiClO.sub.4 in acetonitrile/propylene carbonate (50/50 V/V)
nonaqueous solution. See FIG. 5 for typical CV in nonaqueous
solutions. The cyclic voltammetry experiments are conducted in this
solution on Princeton Applied Research Potentiostat (Model #273)
with potential limits of -0.2 V and 0.8V vs Ag/Ag+ standard
nonaqueous reference. Longevity tests are performed on MACCOR
system with charging the film to 0.8 V with 0.5 mA current rate and
discharging to -0.25 V with 0.5 mA current rate. FIG. 18 shows the
nearly linear nonaqueous behavior of total capacity to total
aqueous coulombic growth of the Al/C/PANi electrode.
Example 3
Negative Electrode
[0124] Due to the instability of common non-aqueous electrolyte
mixtures in the vicinity of high voltage lithium based negative
electrodes, a new slightly lower voltage electrode would be
advantageous. Lithium aluminum alloy has an electrochemical
potential 0.3 Volts less negative than lithium metal or more
importantly the decomposition voltage of propylene carbonate
electrolyte solvent. Aluminum is subject to corrosion and must be
protected from byproducts generated or present in the
electrochemical cell. The following example is a new way to
accomplish a high capacity, high current, chemically protected
negative electrode for use in lithium based cells.
[0125] Step 1. 5052 or other suitable magnesium or other alloy Al
foil, 20 microns thick or so is etched in HCl 1 M solution with 5%
EG in room temperature degassed DI water, 20 mA/cm.sup.2 oxidizing
current for 3 minutes to produce a high surface area substrate. In
this type of electrolytic etch, the thickness of the aluminum foil
remains the same even as the porosity is increased. The weight loss
in this example can be about 40%. The surface area can be increased
by up to a hundred times compared to the bare aluminum foil.
Electrical or ionic currents can be increased evidenced by limited
voltage drop to maintain these currents. Prior to etch, about 0.25
mA can be passed by a 1 cm.sup.2 sample. After etch, as much as 10
ma per square centimeter of substrate area (not accounting for the
etched enhancement) can be passed for the same voltage drop (0.3V
was selected arbitrarily). The micro-etched features minimize
cracks due to lithium based swelling and contraction. It is now
practical to make aluminum foil into high current Li+ source
negative electrode. The foil is then rinsed in DI water for about 5
minutes.
[0126] Step 2. When a lithium cell is charged and discharged, trace
water in the electrolyte may react on the surface of the lithium
donating electrode causing gradual capacity loss. There is a desire
to limit this loss and to otherwise protect the surface of the
aluminum foil. To this end, zinc, tin or other easily oxidized
metal can be plated over the freshly etched aluminum foil. The
plating conditions are NH.sub.4Cl 1 M in degassed DI water with
0.1M ZnCl.sub.2 at 4 mA/cm.sup.2 rate in the amount of 0.025
mAHr/cm.sup.2 or so. The idea is to coat the aluminum with a metal
that will oxidize and then to be preferentially and permanently
reduced by lithium to form a lithium oxide as a protective coating.
The plating should take place immediately after the roughening etch
to enhance the adhesion of the new metal. "Immediately" is defined
as a short enough time to substantially limit Al oxide growth so
that the new metal (Zn, Sn etc.) can plate successfully. The foil
is then rinsed in DI water for about 5 minutes.
[0127] Step 3. The foil is annealed and dried in an IR oven to
oxidize the exposed new metal and to drive off all water. The
temperature is between 100 and 500 degrees C., preferably 100 to
200 in dry gas or -40 degrees dew point air to prepare it for
processing in non-aqueous electrolyte. The aluminum foil is exposed
to this temperature to form a few monolayers of metal oxide on the
surface, and not more which would unnecessarily increase the
irreversible loss of lithium upon the first charge. The time can be
as little as 1 minute in dry air.
[0128] Step 4. The foil is now lithiated in a non-aqueous
electrolyte solution containing a salt of Lithium with or without a
lithium counter electrode. If the salt is LiCl complexed by
AlCl.sub.3 in PC, for example, the only byproduct of a non-lithium
counter electrode setup is Cl.sub.2 gas. The current example uses a
mixture of 1:1 PC and acetonitrile, 1 molar solution of AlCl.sub.3
and 0.1 molar LiCl with a reducing current of 4 mA. In this way, as
production goes on, only additional LiCl needs to be added. This
process takes place in a -40 degrees C. dew point dry space. During
the initial lithiation, any metal (e.g. zinc) oxide will be reduced
by the lithium, and that initial lithium material will be oxidized
into a protective layer of Li.sub.2O. Thereafter, Li+ will permeate
the insoluble Li.sub.2O and lithiate the underlying aluminum (or
other Li active material e.g. zinc, tin, etc). The foil is
lithiated to a minimum amount, in order for it to take on a -2.7
Volt electrochemical potential with respect to an Ag/AgCl reference
electrode. This amount relates to about 0.25 mAHr/cm.sup.2 or about
10% by aluminum atomic count. Irreversible Li losses include both
the minimum aluminum % plus the small Li.sub.2O forming loss. The
reducing current is set to 4 mA but may be lower or as high as 10
mA without altering the Li+ insertion voltage that lies between
-2.7 and -3.0 Volts re. Ag/AgCl.
[0129] When pairing a polyaniline and aluminum foil electrode
together to form an electrochemical cell, it is necessary to
account for lithium ions in particular (since most of the useful
anions are relatively insoluble and don't move far). The diagram on
the left shows how the charge state of a polyaniline positive
electrode determines the level of incorporated lithium ions. The
diagram on the right indicates a "plateau" of at -2.7 Volts for
lithiation percentages between about 10 and 50%. Therefore if the
polyaniline is carrying a charge state of -0.2 and matched to the
negative electrode carrying a minimum level of lithiation, it will
make best utilization of lithium without over-lithiating the
negative electrode, which can be damaging. If the negative
electrode is not lithiated to the minimum value according to the
graph, then some of the lithium originating during the initial
charge will be lost due to the initial irreversible capacity of the
aluminum.
Energy Density
[0130] Ultracapacitor cells with polyaniline positive electrodes
synthesized on nickel and Al--Li negative electrodes have been
progressively optimized using an experimental approach. As shown in
the graph below, subtle changes in combinations of pH and dopant
can result in changes to electrical performance. Shown in FIG. 9
are films A-E that were classified relatively as poor, average, and
promising. Film E and F represents promising results at a certain
pH (adjusted using H.sub.2SO.sub.4) and dopant (CSA) level.
Relative results are explained below: [0131] Film A: Higher pH,
2.times. dopant level: higher pH slows growth and added dopant
grows denser PANI fibers. Film result: poor performance. [0132]
Film B: Lower pH, same dopant level: lower pH speeds film growth
but is destructive to film morphology because not enough dopant is
present. Film result: poor performance. [0133] Film C: Lower pH,
2.times. dopant: lower pH speeds film growth and higher dopant
protects film. Film Result: average film performance. [0134] Film
D: Higher pH, same dopant level: higher pH slows growth. Result:
average film performance. [0135] Film E and Film F: similar pH and
dopant level. However, in Film F the Li--Al electrode was altered
showing improved performance over E.
[0136] Performance variation is seen over a range of variables. The
competition between the dopant acid (CSA) and the general
electrolytic acid (in this test series, H.sub.2SO.sub.4) influences
morphology and electrical performance (as well as longevity, not
shown). Other parameters such as synthesis voltage, temperature,
and transfer conditions are shown to be important.
[0137] The performance characteristics of these synthesized films
are based on the rinsed and dried microbalance weights and
micrometer measured thickness. The thickness of the above reported
films is between 10 and 30 microns, such as about 11 microns, and
the mass is about 0.0009 grams/cm.sup.2. These values of weights
and thickness are typical for the fabrication and testing procedure
used to evaluate growth variables.
[0138] The capacitance measured on discharge curve (F) on FIG. 9
above is about 0.91 F/cm.sup.2, which corresponds to over 1000 F/g
of polyaniline. This value compares favorably with several
electrolytically grown polyaniline nanowire demonstrations on
platinum.
[0139] The electrical and physical data given here is used in the
Computation Model of Specific Energy, Energy Density, Specific
Power and Power presented below. Among other parameters, film
thickness, energy density per square centimeter, and mass are
entered into the model.
[0140] RAW Data at Fast Discharge Rate--can be used to compute
specific power and power density for 30 s at 80% DOD.
[0141] A similar constant current discharge test, performed this
time at 2.5 mA/cm.sup.2, may be used to calculate the "maximum"
power deliverable for 30 seconds commencing with 80% DOD.
[0142] The 30 second interval commencing with 80% DOD (5225-5250
seconds) indicates a power during that time of 7 mw/cm.sup.2.
Preliminary Measurements of Cycle Life
[0143] Accelerated lifetime experiments on the polyaniline
electrode using a platinum substrate show no material degradation
after 25,000 charge-discharge cycles. Below, two overlaid CV curves
demonstrate little degradation after 5000 and 25000 operating
cycles on the aforementioned platinum electrode test.
[0144] A similar accelerated test on a polyaniline electrode using
a modified base metal substrate has been conducted. This CV test,
run at a 4 mV/sec sweep rate, shows little degradation through 850
cycles. In FIG. 12, the lines compare the 50.sup.th and 850.sup.th
cycle. Note that the voltage in this test is shown against an
Ag/AgCl reference.
[0145] The following FIG. 14 describes performance of a half cell
arrangement of polyaniline electrochemically synthesized onto
graphite scrubbed Aluminum 1145. The sweep rates were roughly
equivalent to 15 mv/sec. for 250 cycles, and then one cycle at an
equivalent of 4 mv/sec sweep rate. This half cell test was
continued for over 4000 cycles as shown here. Since peak currents
are proportion to capacity, they are used here to monitor longevity
of the film at the two charge and discharge rates. Over the course
of this test, the peak currents declined by only 21%. We recognize
that, a shift of performance occurred near the center of the test
period, and this may be attributed to a change in the room
temperature of the test lab over the same period. Performance has
increased to 0.8 mAhr/cm.sup.2, which, if discharged against a
suitable negative electrode such as lithiated aluminum resulting in
a full cell average voltage of 3.7 V. Etched, lithiated aluminum
5052 are preferred. Initial cycle tests (over 600) have been
measured in half cell with a silver wire pseudo reference
electrode. Initial measured values of 1 mAhr/cm.sup.2 capacity
exceed that of the positive electrode, and appear sufficient to
support our full cell voltage assumptions.
[0146] While there has been illustrated and described what is at
present considered to be the preferred embodiment of the present
invention, it will be understood by those skilled in the art that
various changes and modifications may be made and equivalents may
be substituted for elements thereof without departing from the true
scope of the invention. Therefore, it is intended that this
invention not be limited to the particular embodiment disclosed,
but that the invention will include all embodiments falling within
the scope of the appended claims.
* * * * *